1. Field of Invention
The present invention relates to an apparatus for adjusting the spectrum and bandwidth of a laser light source.
2. Description of Related Art
Excimer lasers are currently used as light sources for the integrated circuit lithography industry. These lasers produce a beam having a narrowband spectrum with a bandwidth of less than 1 pm at deep ultraviolet (DUV) wavelengths of 248 nm for a KrF laser or 193 nm for an ArF laser. Molecular fluorine lasers emit around 157 nm and will become more widely used for vacuum ultraviolet (VUV) lithography for producing even smaller structures on silicon wafers. There also exist semi-narrowband excimer lasers with bandwidths of more than 10 pm, for which the same principles hold. To produce extremely narrow-band UV light of low divergence and of a high spectral purity, a multitude of dispersive optical components may be utilized such as prisms, optical diffraction gratings and etalons or other interferometric devices. In general, adjustments to the wavelength and/or bandwidth of UV light emitted by these lasers may be made by using an electromechanical device (xe2x80x9cEMDxe2x80x9d), which in some way moves the position or changes the surface curvature of an optical component (xe2x80x9cOCxe2x80x9d) in the resonator of the excimer or molecular flourine laser. The EMD is coupled to a mechano-optical device (xe2x80x9cMODxe2x80x9d) which transfers the motion of the EMD to the optical component, wherein the OC may be typically fixed to the MOD. Thus, when the optical component is moved, characteristics of the UV light output from the laser are changed.
In the normal operation mode of a DUV or VUV lithography laser system, it is desired to keep the laser wavelength substantially constant and the bandwidth (or another spectral property like full width at 1/e2, spectral purity, or an integral or differential quantity) below a specified value. These quantities can be monitored and controlled using a spectrometer such as an etalon spectrometer, grating spectrometer, prism spectrometer or other optical spectrometers in conjunction with a processor in a feedback loop including means for adjusting spectral parameters to desired values.
The typical temporal exposure pattern for the production of semiconductor chips is produced with pulse bursts of, e.g., 200 laser pulses and short breaks of 100 ms between them. A complete sequence may include 60 bursts and breaks, which is followed by a more or less long burst break of, e.g., 5 seconds. For lithography systems it is desired to keep the quality of the laser radiation under control for each pulse of the burst sequence. It is desired to have a lithography system wherein the wavelength of emission may be changed, particularly within a long burst break, over a wide range of up to 300 pm. This is to adapt the lithography system to environmental conditions like pressure and temperature. Furthermore, changes of wavelength in a small range of up to 0.6 pm without any laser pulses being emitted from the laser during that wavelength change are desired within short burst breaks, i.e. with an open feedback loop. This is to keep the quality of the lithography process under control, as changes in temperature of stepper optics can otherwise result in changes in exposure wavelength at an application process.
The desired tolerance limit of such componentry is extremely low in respect of hysteresis. Maintaining a stable and invariable position of optical elements and mechanical componentry within a defined range is, therefore, greatly desired.
It is further desired that the mounting of the OC be independent of environmental conditions, in particular of possible temperature gradients within the optical component. The demands set out above apply also to elements and componentry in motion during operation. These linear and rotary motions can be very small (e.g.,  less than 100 nm:x rad) and are essentially designed to fine tune the entire optical system to a desired wavelength of the UV light. This may result in high acceleration values and it is desired that such values be free of negative influence on the positional stability of the optical elements. Merest inaccuracies already prove undesirable during repeated starts at pre-defined set-points. These set-points can be reference coordinates at which the reference wavelength, for exampled 248.3271 nm, is found. To calculate various operating positions, it is desired that this value be recorded precisely and be maintained reliably. It is further desired that the hysteresis of such motional process be kept very small ensuring that the wavelength drift of the optical assembly is kept as minute as possible.
Spring mount contact pressure plates may be used for securely positioning OCs. A disadvantage is that there is a pointlike exerting of force into the substrates of OCs using this method. Consequently, this may lead to the development of partially irreversible strain birefringence. This causes severe wavefront deformation and striation in the beam profile. Optically acting gratings and etalons having adjustable orientations for controlling the wavelength and bandwidth of emission of the laser may be supported between high surface quality bearings which permit rotation of the OCs. Such systems are susceptible to hysteresis, and it is recognized herein that special consideration should be given to the design of bearing components. Important quantities are diameter of balls, surface quality of bearing components as well as sizing of pressure forces. In general small quantities of silicone-free lubricants are used for lubrication.
When designing components to support OC""s, it is recognized herein that special consideration should be given for temperature gradient-dependent changes in length. Such assemblies may be generally very sensitive to temperature fluctuations reacting with play in bearings and thus producing hysteresis effects when adjusting positions and/or orientations of OC""s. To a certain degree this is influenced by the breakaway friction of bearing components. This is the force necessary to leave the zone of elastic deformation of the bearing components and to proceed into a rotary, progressive motion.
The present invention relates to an apparatus for adjusting, with low hysteresis and high repeatability, the wavelength and/or bandwidth of a laser beam by moving an optical component.
In a particular embodiment, an apparatus for adjusting a position of an optical component within a laser resonator with suppressed hysteresis includes an electromechanical device comprising a drive element including a first contact surface, and a mechano-optical device for supporting the optical component including a second contact surface for contacting the first contact surface. The drive element permits adjustment of an orientation of the mechano-optical device by applying a force to the first contact surface, and thereby for adjusting an orientation of the optical component. The first and second contact surfaces are configured such that the drive element transmits a change of position to the mechano-optical device through a rolling contact between the first contact surface and the second contact surface.
In another embodiment, an apparatus for adjusting a position of an optical component within a laser resonator includes an electromechanical device comprising a drive element including a first contact surface, and a mechano-optical device for supporting the optical component including a second contact surface for contacting the first contact surface. The drive element permits adjustment of the position of the mechano-optical device by applying a force to the first contact surface, and thereby the mechano-optical device adjusts the position of the optical component. The apparatus also includes a controller for error correction of the position of the drive element which controls the electromechanical device, and a position measuring device which measures the position of the drive element. A signal feedback loop provides a signal indicative of the position of the drive element to the controller from the position measuring device. The controller controls the electromechanical device which adjusts the drive element based on the signal provided by the feed back loop.
In another embodiment, an apparatus for adjusting a position of an optical component within a laser resonator includes an electromechanical device comprising a drive element including a first contact surface, and a mechano-optical device for supporting the optical component including a second contact surface for contacting the first contact surface. The drive element permits adjustment the position of the mechano-optical device by applying a force to the first contact surface. The mechano-optical device adjusts the position of the optical component. The apparatus further includes a controller for error correction of the position of the mechano-optical device for controlling the electromechanical device, and a position measuring device which measures the position of the mechano-optical device. The controller and the position measuring device are connected in a feedback loop. The position measuring device provides a signal indicative of a position of the mechano-optical device. The controller controls the electromechanical device based on the signal from the position measuring device, such that the electromechanical device adjusts a position of the drive element and the drive element adjusts a position of the mechano-optical device based on a control signal from the controller.
According to another embodiment, an apparatus for adjusting a position of an optical component within a laser resonator with suppressed hysteresis includes a housing for mounting the optical component therein, and an electromechanical device including a drive member configured to travel in an approximately linear direction, a mechano-optical device rotatably coupled to the housing, and a contact point where the drive member and the mechano-optical device are in contact with each other. The drive member and mechano-optical device are configured for directing the contact point to follow a substantially tangential path relative to the rotation of the mechano-optical device.
In another embodiment, an apparatus for adjusting a position of an optical component within a laser resonator with suppressed hysteresis includes a mechano-optical device for supporting an optical component, and an electromechanical device comprising a drive element coupled to the mechano-optical device by an elastic material. The drive element is moved by the electromechanical device for adjusting an orientation of the mechano-optical device and thereby for adjusting an orientation of the optical component.
According to a further embodiment, an apparatus for adjusting an orientation of an optical component mounted within a laser resonator includes an optical mount for mounting the optical component thereon; and a mechano-optical device coupled by an elastic material to the optical mount. The mechano-optical device is rotationally adjustable for adjusting the orientation of the optical component within the laser resonator.
In another embodiment, an apparatus for adjusting an orientation of an optical component mounted within a laser resonator includes a mechano-optical device for mounting the optical component thereon. The mechano-optical device is rotationally adjustable for adjusting the orientation of the optical component within the laser resonator. The mechano-optical device is rotatable at least approximately about a center of gravity of the combination of the optical component and mechano-optical device.
In a further embodiment, an apparatus for adjusting a position of an optical component within a laser resonator with suppressed hysteresis includes a mechano-optical device for supporting the optical component and having a contact segment, an electromechanical device comprising a drive element coupled to the mechano-optical device, wherein the drive element is moved by the electromechanical device for adjusting an orientation of the mechano-optical device and thereby for adjusting an orientation of the optical component, and at least one spring for coupling the drive element to the contact segment of the mechano-optical device.
In another embodiment, an apparatus for adjusting a position of an optical component within a laser resonator with suppressed hysteresis includes a mechano-optical device for supporting an optical component, and an electromechanical device comprising a drive element magnetically coupled to the mechano-optical device. The drive element is moved by the electromechanical device for adjusting an orientation of the mechano-optical device and thereby for adjusting an orientation of the optical component.
In a further embodiment, an apparatus for adjusting a position of an optical component within a laser resonator with suppressed hysteresis includes a mechano-optical device for supporting the optical component and having an adjustable orientation for adjusting an orientation of the optical component. The optical component is supported on the mechano-optical device by a roller bearing comprising a ruby ball bearing.
In another embodiment, an apparatus for mounting an optical component within a laser resonator includes a housing disposed within the laser resonator and having the optical component mounted therein, the optical component having an axis of rotation defined therethrough, a first ball bearing coupled to a first surface of the optical component, a second ball bearing coupled to a second surface of the optical component, the first and second ball bearings being substantially aligned along said axis of rotation, and a leaf spring coupled to the housing and also to one of the first and second ball bearings for controlling a spacing between the first and second ball bearings such that the spacing is adjustable to a changing dimension of the optical component as a temperature of the optical component changes.
In another embodiment, an apparatus for mounting an optical component within a laser resonator includes a housing disposed within the laser resonator and having the optical component mounted therein, the optical component having an axis of rotation defined therethrough, a first ball bearing coupled to a first surface of the optical component, a second ball bearing coupled to a second surface of the optical component, the first and second ball bearings being substantially aligned along the axis of rotation, and at least one spring coupled to the housing and also to one of the first and second ball bearings for controlling a spacing between the first and second ball bearings such that the spacing is adjustable to a changing dimension of the optical component as a temperature of the optical component changes.
In a further embodiment, an optical mount for mounting an optical component thereon and having an adjustable orientation within a laser resonator with suppressed hysteresis includes a housing for mounting the optical component thereto, a leaf spring and a leaf spring clamp coupled to the housing, and a first ball bearing and a second ball bearing for rotatably supporting the optical component therebetween. The first ball bearing is supported between the leaf spring and the leaf spring clamp in a direction offset from an axis of rotation of the optical component substantially defined through the first and second ball bearings.
In a further embodiment, a hysteresis reducing optical apparatus for a laser system includes an optical component coupled to an upper roller ball and to a lower roller ball, and a housing including a base, a leaf spring clamp and a clamp. The leaf spring attaches the upper rollerball in a sideways fashion to the housing. The clamp attaches the lower rollerball to the housing in a sideways fashion. The clamp fixes the lower bearing in a stationary manner.
According to another embodiment, an optical mount for mounting an optical component thereon within a laser resonator includes a base for supporting the optical component at a first surface, a plano-curved segment for supporting the optical component at a second surface opposite the first surface, the plano-curved segment supporting the optical component by contacting the second surface of the optical component with a planar surface of the plano-curved segment, a leaf spring contacting a curved surface of the plano-concave segment for controlling a force exerted on the optical component by the planar surface of the plano-curved segment, and at least one spring for coupling the leaf spring to the base for controlling a spacing between the first and second ball bearings such that the spacing is adjustable to a changing dimension of the optical component as a temperature of the optical component changes.
An apparatus for adjusting an orientation of an optical component mounted within a laser resonator with suppressed hysteresis includes an optical mount for mounting the optical component thereon, and an electro-mechanical device coupled by a solid link to the optical mount for adjusting an orientation of the optical component within the laser resonator. The solid link is elastically deformable for providing the suppressed hysteresis.
The features mentioned in the subclaims relate to further developments of the solution according to the invention. Further advantages of the invention are found in the following detailed description.